For recruiting, work backwards. D3s recruit 17U (summer before senior year); if he's viable for them, that's when you'll know. If before 17U your son is 6'4" and throwing 90, you'll be hearing earlier and from different schools than if he's 5'10" and has progressed to 80. As others said, if he wants to play in college, he'll have to work at pitching, building strength, etc.
Ask pitching coaches, travel coaches, etc. what they recommend.
As a note, these days D3 pitchers throw 82-88+ heading into college.
"Best interest" depends on what your son wants out of baseball and school. JUCO is also a possibility, for many different reasons.